This invention relates to gas turbines in general, and to integrated coal gasification combined cycle systems in particular.
Integrated gasification combined cycle systems provide flexible operating systems which burn coal in an environmentally acceptable manner. Such systems typically incorporate air blown or oxygen blown gasifiers. For oxygen blown gasifiers, the gasifier optimum pressure is much higher than the fuel pressure required by the gas turbine. In the case of air blown gasifiers, when the power is provided by a motor to compress air for the gasifier, the power requirement lowers the overall plant power output. In the case of oxygen blown gasifiers, power is recovered by installing a power recovery turbine, or turbo expander, in the fuel gas stream and increasing the inlet temperature indirectly using heat exchangers. In accordance with this invention, additional power is recovered by adding a combustor to the expander, and letting the pressure down through the expander and using the power output of the expander to drive a generator or other load.
More specifically, effluent gas from the gasifier is either cooled in a two train syngas cooler (a radiant cooler followed by a convention cooler) or in a quench cooler where water is injected into the cooler. The fuel gas is further treated to remove particulate material. A turbo expander is included to reduce the gas pressure to gas turbine operating pressure, and to recover useful power. Typically, the temperature before the expander needs to be elevated above the cooled temperature for particulate removal and useful heat removal. This has been accomplished in the past by use of heat exchangers and to some extent by including a shift reactor to raise the temperature.